Pressure responsive pilot control valves act to provide a signal when a high or low pressure condition is sensed within the flowline or process to which such pilot valves are operably connected. A high pressure condition may be created, for example, by a line blockage. In such an event, if uncorrected, the well head pressure may exceed the line rating which can result in a line rupture. A low pressure condition may be created when there exists leakage. In either case, it is imperative that the flow within the flowline be terminated to prevent costly and potentially hazardous spills and to also prevent damage to equipment.
Conventionally, three line pilot valves have been used to sense pressure changes which are outside of desired operating ranges. Such pressure changes create a change in the operating configuration of the three line pilot such that a signal is forwarded and a shut-in valve is operated to close the flowline and terminate flow. Such a three way pilot valve is disclosed, for example, in our copending U.S. application Ser. No. 772,221 filed Oct. 4, 1991 (Ellett).
Inherently, however, three line pilots are more complex and have parts which are otherwise unnecessary in a two line pilot. However, it is important in two line pilots that the set points for fluid exit remain consistent over time and, so far as possible, are independent of the fluid pressure in the inlet chamber of the valve. The set points should change only when a spring is adjusted to deliberately change the set points.